Herpes simplex virus, early neuroimaging markers and incidence of Alzheimer's disease

Microbial infection promotes amyloid pathology in a mouse model of Alzheimer's disease via modulating γ-secretase

Microbial infection as a type of environmental risk factors is considered to be associated with long-term increased risk of dementia, including Alzheimer's disease (AD). AD is characterized by two neuropathologically molecular hallmarks of hyperphosphorylated tau and amyloid-β (Aβ), the latter generated by several biochemically reactive enzymes, including γ-secretase. However, how infectious risk factors contribute to pathological development of the AD core molecules remains to be addressed. In this work, we utilized a modified herpes simplex virus type 1 (mHSV-1) and found that its hippocampal infection locally promotes Aβ pathology in 5 × FAD mice, the commonly used amyloid model. Mechanistically, we identified HSV-1 membrane glycoprotein US7 (Envelope gI) that interacts with and modulates γ-secretase and consequently facilitates Aβ production. Furthermore, we presented evidence that adenovirus-associated virus-mediated locally hippocampal overexpression of the US7 aggravates Aβ pathology in 5 × FAD mice. Collectively, these findings identify a herpesviral factor regulating γ-secretase in the development and progression of AD and represent a causal molecular link between infectious pathogens and neurodegeneration.

Single-genome analysis reveals a heterogeneous association of the herpes simplex virus genome with H3K27me2 and the reader PHF20L1 following infection of human fibroblasts

The fate of herpesvirus genomes following entry into different cell types is thought to regulate the outcome of infection. For the Herpes simplex virus 1 (HSV-1), latent infection of neurons is characterized by association with repressive heterochromatin marked with Polycomb silencing-associated lysine 27 methylation on histone H3 (H3K27me). However, whether H3K27 methylation plays a role in repressing lytic gene expression in non-neuronal cells is unclear. To address this gap in knowledge, and with consideration that the fate of the viral genome and outcome of HSV-1 infection could be heterogeneous, we developed an assay to quantify the abundance of histone modifications within single viral genome foci of infected fibroblasts. Using this approach, combined with bulk epigenetic techniques, we were unable to detect any role for H3K27me3 during HSV-1 lytic infection of fibroblasts. By contrast, we could detect the lesser studied H3K27me2 on a subpopulation of viral genomes, which was consistent with a role for H3K27 demethylases in promoting lytic gene expression. In addition, viral genomes co-localized with the H3K27me2 reader protein PHF20L1, and this association was enhanced by inhibition of the H3K27 demethylases UTX and JMJD3. Notably, targeting of H3K27me2 to viral genomes was enhanced following infection with a transcriptionally defective virus in the absence of Promyelocytic leukemia nuclear bodies. Collectively, these studies implicate a role for H3K27me2 in fibroblast-associated HSV genome silencing in a manner dependent on genome sub-nuclear localization and transcriptional activity.

IMPORTANCE

Investigating the potential mechanisms of gene silencing for DNA viruses in different cell types is important to understand the differential outcomes of infection, particularly for viruses like herpesviruses that can undergo distinct types of infection in different cell types. In addition, investigating chromatin association with viral genomes informs on the mechanisms of epigenetic regulation of DNA processes. However, there is a growing appreciation for heterogeneity in the outcome of infection at the single cell, and even single viral genome, level. Here we describe a novel assay for quantifying viral genome foci with chromatin proteins and show that a portion of genomes are targeted for silencing by H3K27me2 and associate with the reader protein PHF20L1. This study raises important questions regarding the mechanism of H3K27me2-specific targeting to viral genomes, the contribution of epigenetic heterogeneity to herpesvirus infection, and the role of PHF20L1 in regulating the outcome of DNA virus infection.

Update on modifiable risk factors for Alzheimer's disease and related dementias

PURPOSE OF REVIEW

All human beings undergo a lifelong cumulative exposure to potentially preventable adverse factors such as toxins, infections, traumatisms, and cardiovascular risk factors, collectively termed exposome. The interplay between the individual's genetics and exposome is thought to have a large impact in health outcomes such as cancer and cardiovascular disease. Likewise, a growing body of evidence is supporting the idea that preventable factors explain a sizable proportion of Alzheimer's disease and related dementia (ADRD) cases.

RECENT FINDINGS

Here, we will review the most recent epidemiological, experimental preclinical, and interventional clinical studies examining some of these potentially modifiable risk factors for ADRD. We will focus on new evidence regarding cardiovascular risk factors, air pollution, viral and other infectious agents, traumatic brain injury, and hearing loss.

SUMMARY

While greater and higher quality epidemiological and experimental evidence is needed to unequivocally confirm their causal link with ADRD and/or unravel the underlying mechanisms, these modifiable risk factors may represent a window of opportunity to reduce ADRD incidence and prevalence at the population level via health screenings, and education and health policies.

Infection burden and its association with neurite orientation dispersion and density imaging markers in the UK Biobank

BACKGROUND

Infection burden (IB), although linked to neurodegeneration, including Alzheimer's Disease (AD), has not been examined against neurite orientation, dispersion, and density imaging (NODDI) measures.

METHODS

Among 38,803 UK Biobank adults (Age:40-70 years), we tested associations of total IB (IBtotal, 47.5 %) and hospital-treated IB (IBhosp, 9.7 %) with NODDI measures (5-15 years later), including volume fraction of Gaussian isotropic diffusion (ISOVF), intra-cellular volume fraction (ICVF) and orientation dispersion (OD) indices, using multiple linear regression models.

RESULTS

Total and hospital-treated infection burdens (IBtotal and IBhosp) were associated with increased ISOVF, indicating increased free-water component. IBtotal was positively associated with OD, indicating that at higher IBtotal there was greater fanning of neurites. This was more evident in the lower cardiovascular health group. IBhosp was associated with higher OD, and lower ICVF at higher AD polygenic risk. Together, these findings indicate that both total and hospital-treated infections have effects on NODDI outcomes in the direction of poor brain health. These effects were largely homogeneous across cardiovascular health and AD polygenic risk groups, with some effects shown to be stronger at poor cardiovascular health and/or higher AD risk.

CONCLUSIONS

Total and hospital-treated infections were associated with poorer white matter microstructure (higher ISOVF or OD or lower ICVF), with some heterogeneity across cardiovascular health and AD risk. Longitudinal studies with multiple repeats on neuroimaging markers in comparable samples are needed.

Herpes Simplex Viral Infection Doubles the Risk of Dementia in a Contemporary Cohort of Older Adults: A Prospective Study

Background

Evidence indicates that herpes simplex virus (HSV) participates in the pathogenesis of Alzheimer's disease (AD).

Objective

We investigated AD and dementia risks according to the presence of herpesvirus antibodies in relation to anti-herpesvirus treatment and potential APOE ɛ4 carriership interaction.

Methods

This study was conducted with 1002 dementia-free 70-year-olds living in Sweden in 2001-2005 who were followed for 15 years. Serum samples were analyzed to detect anti-HSV and anti-HSV-1 immunoglobulin (Ig) G, anti-cytomegalovirus (CMV) IgG, anti-HSV IgM, and anti-HSV and anti-CMV IgG levels. Diagnoses and drug prescriptions were collected from medical records. Cox proportional-hazards regression models were applied.

Results

Cumulative AD and all-cause dementia incidences were 4% and 7%, respectively. Eighty-two percent of participants were anti-HSV IgG carriers, of whom 6% received anti-herpesvirus treatment. Anti-HSV IgG was associated with a more than doubled dementia risk (fully adjusted hazard ratio = 2.26, p = 0.031). No significant association was found with AD, but the hazard ratio was of the same magnitude as for dementia. Anti-HSV IgM and anti-CMV IgG prevalence, anti-herpesvirus treatment, and anti-HSV and -CMV IgG levels were not associated with AD or dementia, nor were interactions between anti-HSV IgG and APOE ɛ4 or anti-CMV IgG. Similar results were obtained for HSV-1.

Conclusions

HSV (but not CMV) infection may be indicative of doubled dementia risk. The low AD incidence in this cohort may have impaired the statistical power to detect associations with AD.

Single-genome analysis reveals heterogeneous association of the Herpes Simplex Virus genome with H3K27me2 and the reader PHF20L1 following infection of human fibroblasts

The fate of herpesvirus genomes following entry into different cell types is thought to regulate the outcome of infection. For the Herpes simplex virus 1 (HSV-1), latent infection of neurons is characterized by association with repressive heterochromatin marked with Polycomb silencing-associated lysine 27 methylation on histone H3 (H3K27me). However, whether H3K27 methylation plays a role in repressing lytic gene expression in non-neuronal cells is unclear. To address this gap in knowledge, and with consideration that the fate of the viral genome and outcome of HSV-1 infection could be heterogeneous, we developed an assay to quantify the abundance of histone modifications within single viral genome foci of infected fibroblasts. Using this approach, combined with bulk epigenetic techniques, we were unable to detect any role for H3K27me3 during HSV-1 lytic infection of fibroblasts. In contrast, we could detect the lesser studied H3K27me2 on a subpopulation of viral genomes, which was consistent with a role for H3K27 demethylases in promoting lytic gene expression. This was consistent with a role for H3K27 demethylases in promoting lytic gene expression. In addition, viral genomes co-localized with the H3K27me2 reader protein PHF20L1, and this association was enhanced by inhibition of the H3K27 demethylases UTX and JMJD3. Notably, targeting of H3K27me2 to viral genomes was enhanced following infection with a transcriptionally defective virus in the absence of Promyelocytic leukemia nuclear bodies. Collectively, these studies implicate a role for H3K27me2 in fibroblast-associated HSV genome silencing in a manner dependent on genome sub-nuclear localization and transcriptional activity.

Immunotherapies Targeting Amyloid and Tau Protein in Alzheimer's Disease: Should We Move Away from Diseases and Focus on Biological Targets? A Systematic Review and Expert Opinion

INTRODUCTION

Alzheimer's disease (AD) is the most common cause of dementia worldwide, making it a major public health issue. Anti-amyloid and anti-tau antibodies are the most advanced therapeutic approach at present. Three drugs (lecanemab, donanemab and aducanumab) are on track to be marketed in the coming months. In this systematic review, we review all Phase 2 and Phase 3 clinical trials conducted in this indication and the particularities of the molecules tested.

METHODS

The PubMed and ClinicalTrials.gov databases were searched through February 2023 for Phase 2 and 3 clinical trials involving passive anti-amyloid or anti-tau immunotherapies with published results. This review has been compiled in compliance with the PRISMA checklists.

RESULTS

Of the 165 studies found and after eliminating duplicates, 40 studies had their results published on PubMed and/or ClinicalTrials.gov. Eight anti-amyloid molecules and four anti-tau molecules were the subject of Phase 2 studies, seven anti-amyloids were the subject of Phase 3 trials, and two molecules were granted early marketing approval by the US Food and Drug Administration (FDA). The results were compiled in summary tables showing the primary endpoints used, results, age of the study population and specific adverse events for these molecules.

DISCUSSION

Passive immunotherapy in AD is largely dominated by anti-amyloid antibodies, which are more numerous and more advanced in the pipeline. Lecanemab, donanemab and aducanumab are distinguished by their relative efficacy in terms of cognitive and functional evaluation but also by a decrease in amyloid and tau proteins in the brain. These three molecules have in common that they bind to N-terminal ends of amyloid fibrils and plaques. The findings of their studies raise the question of which criteria to apply when choosing which patient will receive them when marketed, such as the apoliprotein E gene's fourth allele (APOE4) genetic status of patients. The large number of negative studies may also raise the question of the criteria for defining the disease and the possible interest in redefining it on biological grounds to offer a more personalized medicine to patients suffering from neurodegenerative diseases.

Cardiovascular health, infection burden and their interactive association with brain volumetric and white matter integrity outcomes in the UK Biobank

BACKGROUND

Cardiovascular health is associated with brain magnetic resonance imaging (MRI) markers of pathology and infections may modulate this association.

METHODS

Using data from 38,803 adults (aged 40-70 years) and followed-up for 5-15 years, we tested associations of prevalent total (47.5%) and hospital-treated infection burden (9.7%) with brain structural and diffusion-weighted MRI (i.e., sMRI and dMRI, respectively) common in dementia phenome. Poor white matter tissue integrity was operationalized with lower global and tract-specific fractional anisotropy (FA) and higher mean diffusivity (MD). Volumetric sMRI outcomes included total, gray matter (GM), white matter (WM), frontal bilateral GM, white matter hyperintensity (WMH), and selected based on previous associations with dementia. Cardiovascular health was measured with Life's Essential 8 score (LE8) converted to tertiles. Multiple linear regression models were used, adjusting for intracranial volumes (ICV) for subcortical structures, and for demographic, socio-economic, and the Alzheimer's Disease polygenic risk score for all outcomes, among potential confounders.

RESULTS

In fully adjusted models, hospital-treated infections were inversely related to GM (β ± SE: -1042 ± 379, p = 0.006) and directly related to WMH as percent of ICV (Loge transformed) (β ± SE:+0.026 ± 0.007, p < 0.001). Both total and hospital-treated infections were associated with poor WMI, while the latter was inversely related to FA within the lowest LE8 tertile (β ± SE:-0.0011 ± 0.0003, p < 0.001, PLE8×IB < 0.05), a pattern detected for GM, Right Frontal GM, left accumbens and left hippocampus volumes. Within the uppermost LE8 tertile, total infection burden was linked to smaller right amygdala while being associated with larger left frontal GM and right putamen volumes, in the overall sample. Within that uppermost tertile of LE8, caudate volumes were also positively associated with hospital-treated infections.

CONCLUSIONS

Hospital-treated infections had more consistent deleterious effects on volumetric and white matter integrity brain neuroimaging outcomes compared with total infectious burden, particularly in poorer cardiovascular health groups. Further studies are needed in comparable populations, including longitudinal studies with multiple repeats on neuroimaging markers.

Mechanistic insights into the role of herpes simplex virus 1 in Alzheimer's disease

Alzheimer's Disease (AD) is an aging-associated neurodegenerative disorder, threatening millions of people worldwide. The onset and progression of AD can be accelerated by environmental risk factors, such as bacterial and viral infections. Human herpesviruses are ubiquitous infectious agents that underpin numerous inflammatory disorders including neurodegenerative diseases. Published studies concerning human herpesviruses in AD imply an active role HSV-1 in the pathogenesis of AD. This review will summarize the current understanding of HSV-1 infection in AD and highlight some barriers to advance this emerging field.

Longitudinal Effects of Herpesviruses on Multiple Cognitive Outcomes in Healthy Elderly Adults

BACKGROUND

Herpesviruses have been proposed to be involved in Alzheimer's disease development as potentially modifiable pathology triggers.

OBJECTIVE

To investigate associations of serum antibodies for herpes simplex virus (HSV)-1 and cytomegalovirus (CMV) and anti-herpesvirus treatment with cognitive outcomes in relation to interactions with APOE ɛ4.

METHODS

The study included 849 participants in the population-based Prospective Investigation of the Vasculature in Uppsala Seniors study. Cognitive performance at the ages of 75 and 80 years was assessed using the Mini-Mental State Examination (MMSE), trail-making test (TMT) A and B, and 7-minute screening test (7MS).

RESULTS

Anti- HSV-1 IgG positivity was associated cross-sectionally with worse performance on the MMSE, TMT-A, TMT-B, 7MS, enhanced free recall, and verbal fluency tests (p = 0.016, p = 0.016, p <  0.001, p = 0.001, p = 0.033, and p <  0.001, respectively), but not orientation or clock drawing. Cognitive scores did not decline over time and longitudinal changes did not differ according to HSV-1 positivity. Anti- CMV IgG positivity was not associated cross-sectionally with cognition, but TMT-B scores declined more in anti- CMV IgG carriers. Anti- HSV-1 IgG interacted with APOE ɛ4 in association with worse TMT-A and better enhanced cued recall. Anti- HSV IgM interacted with APOE ɛ4 and anti-herpesvirus treatment in association with worse TMT-A and clock drawing, respectively.

CONCLUSION

These findings indicate that HSV-1 is linked to poorer cognition in cognitively healthy elderly adults, including impairments in executive function, memory, and expressive language. Cognitive performance did not decline over time, nor was longitudinal decline associated with HSV-1.

Recent insights into viral infections as a trigger and accelerator in alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder for which only symptomatic medication is available, except for the recently FDA-approved aducanumab. This lack of effective treatment urges us to investigate alternative paths that might contribute to disease development. In light of the recent SARS-CoV-2 pandemic and the disturbing neurological complications seen in some patients, it is desirable to (re)investigate the viability of the viral infection theory claiming that a microbe could affect AD initiation and/or progression. Here, we review the most important evidence for this theory with a special focus on two viruses, namely HSV-1 and SARS-CoV-2. Moreover, we discuss the possible involvement of extracellular vesicles (EVs). This overview will contribute to a more rational approach of potential treatment strategies for AD patients.

Herpes simplex virus and rates of cognitive decline or whole brain atrophy in the Dominantly Inherited Alzheimer Network

OBJECTIVE

To investigate whether herpes simplex virus type 1 (HSV-1) infection was associated with rates of cognitive decline or whole brain atrophy among individuals from the Dominantly Inherited Alzheimer Network (DIAN).

METHODS

Among two subsets of the DIAN cohort (age range 19.6-66.6 years; median follow-up 3.0 years) we examined (i) rate of cognitive decline (N = 164) using change in mini-mental state examination (MMSE) score, (ii) rate of whole brain atrophy (N = 149), derived from serial MR imaging, calculated using the boundary shift integral (BSI) method. HSV-1 antibodies were assayed in baseline sera collected from 2009-2015. Linear mixed-effects models were used to compare outcomes by HSV-1 seropositivity and high HSV-1 IgG titres/IgM status.

RESULTS

There was no association between baseline HSV-1 seropositivity and rates of cognitive decline or whole brain atrophy. Having high HSV-1 IgG titres/IgM was associated with a slightly greater decline in MMSE points per year (difference in slope - 0.365, 95% CI: -0.958 to -0.072), but not with rate of whole brain atrophy. Symptomatic mutation carriers declined fastest on both MMSE and BSI measures, however, this was not influenced by HSV-1. Among asymptomatic mutation carriers, rates of decline on MMSE and BSI were slightly greater among those who were HSV-1 seronegative. Among mutation-negative individuals, no differences were seen by HSV-1. Stratifying by APOE4 status yielded inconsistent results.

INTERPRETATION

We found no evidence for a major role of HSV-1, measured by serum antibodies, in cognitive decline or whole brain atrophy among individuals at high risk of early-onset AD.

Herpes Viruses in the Baltimore Longitudinal Study of Aging: Associations With Brain Volumes, Cognitive Performance, and Plasma Biomarkers

BACKGROUND AND OBJECTIVES

Although an infectious etiology of Alzheimer disease (AD) has received renewed attention with a particular focus on herpes viruses, the longitudinal effects of symptomatic herpes virus (sHHV) infection on brain structure and cognition remain poorly understood, as does the effect of sHHV on AD/neurodegeneration biomarkers.

METHODS

We used a longitudinal, community-based cohort to characterize the association of sHHV diagnoses with changes in 3 T MRI brain volume and cognitive performance. In addition, we related sHHV to cross-sectional differences in plasma biomarkers of AD (β-amyloid [Aβ]42/40), astrogliosis (glial fibrillary acidic protein [GFAP]), and neurodegeneration (neurofilament light [NfL]). Baltimore Longitudinal Study of Aging participants were recruited from the community and assessed with serial brain MRIs and cognitive examinations over an average of 3.4 (SD = 3.2) and 8.6 (SD = 7.7) years, respectively. sHHV classification used International Classification of Diseases, Ninth Revision codes documented at comprehensive health and functional screening evaluations at each study visit. Linear mixed-effects and multivariable linear regression models were used in analyses.

RESULTS

A total of 1,009 participants were included in the primary MRI analysis, 98% of whom were cognitively normal at baseline MRI (mean age = 65.7 years; 54.8% female). Having a sHHV diagnosis (N = 119) was associated with longitudinal reductions in white matter volume (annual additional rate of change -0.34 cm3/y; p = 0.035), particularly in the temporal lobe. However, there was no association between sHHV and changes in total brain, total gray matter, or AD signature region volumes. Among the 119 participants with sHHV, exposure to antiviral treatment attenuated declines in occipital white matter (p = 0.04). Although the sHHV group had higher cognitive scores at baseline, sHHV diagnosis was associated with accelerated longitudinal declines in attention (annual additional rate of change -0.01 Z-score/year; p = 0.008). In addition, sHHV diagnosis was associated with elevated plasma GFAP, but not related to Aβ42/40 and NfL levels.

DISCUSSION

These findings suggest an association of sHHV infection with white matter volume loss, attentional decline, and astrogliosis. Although the findings link sHHV to several neurocognitive features, the results do not support an association between sHHV and AD-specific disease processes.

Neuroinflammation in neurodegeneration via microbial infections

Recent epidemiological studies show a noticeable correlation between chronic microbial infections and neurological disorders. However, the underlying mechanisms are still not clear due to the biological complexity of multicellular and multiorgan interactions upon microbial infections. In this review, we show the infection leading to neurodegeneration mediated by multiorgan interconnections and neuroinflammation. Firstly, we highlight three inter-organ communications as possible routes from infection sites to the brain: nose-brain axis, lung-brain axis, and gut-brain axis. Next, we described the biological crosstalk between microglia and astrocytes upon pathogenic infection. Finally, our study indicates how neuroinflammation is a critical player in pathogen-mediated neurodegeneration. Taken together, we envision that antibiotics targeting neuro-pathogens could be a potential therapeutic strategy for neurodegeneration.

Antiherpetic drugs: a potential way to prevent Alzheimer's disease?

Background

Considering the growing body of evidence suggesting a potential implication of herpesviruses in the development of dementia, several authors have questioned a protective effect of antiherpetic drugs (AHDs) which may represent a new means of prevention, well tolerated and easily accessible. Subsequently, several epidemiological studies have shown a reduction in the risk of dementia in subjects treated with AHDs, but the biological plausibility of this association and the impact of potential methodological biases need to be discussed in more depth.

Methods

Using a French medico-administrative database, we assessed the association between the intake of systemic AHDs and the incidence of (i) dementia, (ii) Alzheimer’s disease (AD), and (iii) vascular dementia in 68,291 subjects over 65 who were followed between 2009 and 2017. Regarding potential methodological biases, Cox models were adjusted for numerous potential confounding factors (including proxies of sociodemographic status, comorbidities, and use of healthcare) and sensitivity analyses were performed in an attempt to limit the risk of indication and reverse causality biases.

Results

9.7% of subjects (n=6642) had at least one intake of systemic AHD, and 8883 incident cases of dementia were identified. Intake of at least one systemic AHD during follow-up was significantly associated with a decreased risk of AD (aHR 0.85 95% confidence interval [0.75–0.96], p=0.009) and, to a lesser extent with respect to p values, to both dementia from any cause and vascular dementia. The association with AD remained significant in sensitivity analyses. The number of subjects with a regular intake was low and prevented us from studying its association with dementia.

Conclusions

Taking at least one systemic AHD during follow-up was significantly associated with a 15% reduced risk of developing AD, even after taking into account several potential methodological biases. Nevertheless, the low frequency of subjects with a regular intake questions the biological plausibility of this association and highlights the limits of epidemiological data to evaluate a potential protective effect of a regular treatment by systemic AHDs on the incidence of dementia

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-021-00950-0.